Asbestos Toxicology

There is some understanding of the toxic mechanisms, but the basis for mesothelioma formation is unknown

Asbestos is inert in natural rock form. For it to be hazardous, it must be turned into fine flakes of dust that can be inhaled. So there is a risk in miniing, milling, and abatement activities with friability.

Respirators make effective safety devices. However, dust and fibers go home on the clothing of workers. Self exposure, family exposure: there is a ten times increased risk of mesothelioma in women who live with asbestos workers.

Science has shown that the hazard depends on particle size.

Length of fibers: 2 microns: asbestosis
5 microns: mesothelioma, asbestosis
10 microns: bronchogenic lung cancers associated with cigarette smoking and asbestosis.

Diameter:

Over 3 microns: no mesothelioma
Less than 0.5 microns: mesothelioma

The toxic effects of asbestos are not due to its chemical make-up, but its structural features: fibers.

Chrysotile is 90% of the world’s asbestos production. Amosite appears to be the culprit. 5-10% amphiboles which are rod forms associated with malignant mesothelioma even when chrysotile is present.

Chyrsotile breaks down in the body much faster than amphiboles. There is an association between malignant mesothelioma and chrysotile when chrysotile is present at 400 times the lung burden of amosite.

One interesting thing is that animals are affected differently from humans to asbestos fibers. Chrysotile appears to be a more potent inducer of mesothelioma than amphibole fibers. The two types of asbestos break down at similar rates in rodents. Interestingly, other fibers such as fiberglass are carcinogens in rats but not in humans. Researchers speculate that fiber types may differ in their ability to catalyze reactions between iron ions and oxygen. This reaction could cause free radicals that lead to lipid peroxidation, a fibrosis response, DNA damage, and mutations and cancer.

Synergy

Smoking has been hypothesized to have a synergistic effect with asbestos in causing mesothelioma as has arsenic, which is often found in the same workplace settings.

Asbestos is mined and milled for it’s insulating and fireproofing properties. It was sued for electrical insulation, brake linings, floor tiles, transmissions, high pressure sprays to coat outer surfaces of major construction, etc. The first widespread use was in 1913. In the 1940s the shipbuilding industry employed heavy amounts of asbestos. In the late 1960s, use soared as asbestos found use in rocket engines, paper and cement products, pipe wrapping, ceiling tiles, gaskets, hair dryers, textiles, and potholders. By 1973 world consumption was up to 4 million tons per year.

There are three major diseases associated with exposure:

1) Asbestosis – fibrosis of the lung alveoli
2) Bronchogenic lung cancers
3) Malignant mesothelioma – cancer of the pleura (lung lining) and cancer of the peritoneam (lining of the abdominal cavity)

Epidemiology History

Miners were known to get asbestosis in the 1920s. In the 1960s doctors in South Africa noticed high rates of a new form of cancer in miners: malignant mesothelioma.

A study by Blot, et.al., showed that shipyward workers from World War II, who were exposed for less than 5 years, continued to be at risk for mesothelioma over 30 years later. The latency period is high.